Fiber mat forming systems equipped with a lickerin in a top end of a diffuser chute are usually limited in capacity to about 500 kg/hr to produce a web thickness of about 5 cm, and this is due primarily to the formation width, in the machine direction, of the forming surface. This formation width is limited to about 20 cm because the airflow within the chute cannot be expanded from a small width at the lickerin level without affecting the uniformity of the web formation. Too large an expansion in the chute creates airflow boundary layer separation, that is to say, airflow deviation to a side wall of the chute, and this has an adverse effect on the distribution of fibers and the formation profile across the machine direction. It has been found that the problem of boundary layer separation is severely increased when the forming length, that is to say, the length across the machine direction, is larger than the lickerin length.
We have found from experiments that a major cause of improper lay-down of fibers on the forming belt is due primarily to the presence of stalled airflow or separated airflows near the entrance portion of the chute or diffuser. Experimentation has also shown that such deviated airflow within the diffuser duct also cause increased static pressure in the flow-wise direction within the duct, and hence produces boundary layer separation.
Through computational fluid dynamics modeling techniques we have established that steep gradients of static pressure are present in the upper part of the diffuser, and recovery to near atmospheric pressure is achieved only very close to the forming surfaces in the bottom of the diffuser. Such modeling techniques have also revealed that flow separation starts in the corners or the duct at the top end and grows on the side wall opposite the top end of the forming belt. Flow separation causes clumping and non-uniform lay-down of fibers on the forming belt, and this is undesirable as the fiber mat produced does not have uniform density.
The above noted boundary layer formation problem was researched and it was observed in the literature dealing with diffusing wind tunnels, where such problems also occur, that a possible solution is to apply suction in the duct in the area of turbulence or boundary layer formation to ensure that the airflow remained attached to the walls of the tunnel. Such solutions are discussed in an article entitled, "Structure of a separating turbulent boundary layer", Part 1, Mean Flow and Reynold Stresses, published in the Journal of Fluid Mechanics, Vol. 113, 1981, pp. 23-51, Simpson et al. Such solution however posed difficult problems when dealing with forming diffusers in which the air stream contains fibers of different types and which may be in admixture with powdered glue or other products.